My father built a Roberts 58 in Seattle Wa and she was launched in 1990. I am having to repower the main engine and really need some quick help and advice on what size engine to replace the old DH100 Isuzu 190HP that my father installed and purchased from MER Equipment in Seattle. I will give you some background information to help you understand what we are dealing with.fficeffice" />>>>>The boat was a one off Airex foam core hull and was built to the heaviest standards. We have 25,000 lbs of lead in the keel and hold 900 gallons of water and 900 gallons of fuel. I was told the recommended engine size was 90 – 120HP but my father always went overboard and installed a 190HP with a Capitol 3 to 1 transmission turning a 2 ˝” shaft to a 28” (we think) prop. We estimate the boat to weigh between 65K and 70K so she is heavy. I motor sailed the boat down to the Caribbean 3 years ago. We are only able to get 6 to 7 knots at 1500 RPM with the current engine and when we go higher in RPM the boat makes a grinding sound in the shaft near the back where the shaft exits the hull. I know (to the best of my knowledge) that the shaft is straight and the carrier bearing is good so the noise could be coming from the prop being mis-sized or the pitch of the prop. I am told the boat should do 10 to 11 knots on the hull speed. I have had the reps at Yanmar say I can use a 200HP 4LHA-DTP engine with a 2.8 to 1 transmission to turn the 28” prop. I am concerned that to get the 200HP I will need to run the engine at over 2800PRM and may have the same problem I am having now. I am told I need to start with what prop is designed to be used with this hull and go forward from there. >>>>Can you tell me what engine HP and prop size has been used on similar boats and what you would recommend we use now. I really need this answer ASAP as I need to order the new engine by Monday to get it shipped here in time. I will include a picture of the boat out of the water and the prop currently being used. Let me know if there is a number I can call to talk about this.>>

Good Question, we never went above 1800 to 2000 RPM and didnt seem to get much more speed. Here is what Bruce Roberts Austraila had to say

"Re the statement 'I am told I need to start with what prop is designed to be used with this hull and go forward from there.' This is not really correct. With the large variance in displacement with home built vessels and the engine the builder can get the best deal on we work out on a individual basis the reduction gear and prop best suited for the vessel as part of ongoing service. Bruce may do the same. The hull speed of the HM 57 is around 10 to 11 kts and it would amaze me if the grinding noise you hear is from the prop."

This does not really help me figure out what size prop I should use.fficeffice" />>>

Rule of thumb is about 2 HP per 1000 lbs displacement, so 150 HP should be enough. The correct prop pitch is critical in being able to actually operate the engine at rated power and RPM. Two much pitch and the engine will bog down and won't reach rated RPM and power. Two little and it reaches max RPM at less than rated torque and therefore less than rated power. I would contact a couple prop manufacturers to see what they recommend for size and see if your current prop is close. Note that the transmission reduction is also a factor as it affects the prop RPM at a given engine speed.

So how fast did the boat go in 1990 with the 190 HP engine and current trans and prop?

Going to 200 HP and still only running at 1800 RPM because something behind the engine is making a grinding noise will not get you any more speed.

I guess installing a 400 HP engine might make 200 HP at 1800 RPM but to get 10 kts. you would have to double the pitch of the prop wich would probably need 400 HP at 1800 RPM and to get that you might need at least 800 HP.

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